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Re: 7 logical-xor implementations in source tree
From: |
Mattias Engdegård |
Subject: |
Re: 7 logical-xor implementations in source tree |
Date: |
Tue, 23 Jul 2019 18:41:09 +0200 |
23 juli 2019 kl. 14.38 skrev Stefan Monnier <address@hidden>:
>
> I'm not convinced it's worth the trouble (we only have 7 uses so far
> and they all seem happy with a 2-arg xor).
> Especially since there are 2 different reasonable semantics.
> It can always be extended later if needed.
That's reasonable. Racket made it 2-arg (thanks for the reference, Basil);
looks like they couldn't make up their minds either.
>> * Give it a compiler macro, for efficient partial application
>
> Given how rarely it's used, I'm not sure it's worth the trouble.
> Luckily if we only accept the 2-args case this can be done very cheaply
> with defsubst or define-inline.
Yes, define-inline in particular will produce decent code.
I would probably use the negation, boolean equivalence, more often than xor
myself.
Since `equiv' (placeholder name) is more readily thought of as an equivalence
predicate, its n-ary semantics is subject to less debate.
Example implementations:
+(define-inline xor (arg1 arg2)
+ "Boolean exclusive-or: the non-nil argument if the other is nil, else nil."
+ (inline-letevals (arg1 arg2)
+ (inline-quote
+ (if ,arg1
+ (if ,arg2
+ nil
+ ,arg1)
+ ,arg2))))
+
+(defmacro equiv (&rest args)
+ "Boolean equivalence: t if arguments are all non-nil or all nil."
+ (cond ((null args) t)
+ ((null (cdr args)) `(progn ,(car args) t))
+ (t `(if ,(car args)
+ (and ,@(append (cdr args) '(t)))
+ (not (or ,@(cdr args)))))))
- Re: 7 logical-xor implementations in source tree, (continued)
- Re: 7 logical-xor implementations in source tree, Andreas Schwab, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Andy Moreton, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Basil L. Contovounesios, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Andy Moreton, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Basil L. Contovounesios, 2019/07/24
- Re: 7 logical-xor implementations in source tree, Basil L. Contovounesios, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Yuri Khan, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Juri Linkov, 2019/07/25
- Re: 7 logical-xor implementations in source tree, Noam Postavsky, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Stefan Monnier, 2019/07/23
- Re: 7 logical-xor implementations in source tree,
Mattias Engdegård <=
- Re: 7 logical-xor implementations in source tree, Eli Zaretskii, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Paul Eggert, 2019/07/23
- Re: 7 logical-xor implementations in source tree, Stefan Monnier, 2019/07/23